1. Field of the Invention
The present invention relates to a process for the recovery of vanadium from wet-process phosphoric acid and more particularly to a simultaneous oxidation-extraction step in the initial stages of the DEPA-TOPO and DOPPA-TOPO processes for such recovery.
2. Prior Art
In the mining of phosphate rock to recover phosphate-containing fertilizer, the first step consists of digesting the rock with sulfuric acid to produce a phosphoric acid solution and calcium sulfate. The crude phosphoric acid solution, called "wet-process" phosphoric acid, can be processed to remove vanadium and uranium values, particularly if it can be economically justified. Recovery of vanadium and uranium from crude phosphoric acid not only provides an additional valuable mineral source, but also removes unwanted contaminants from the phosphoric acid, much of which is used to make fertilizer. It is important, however, that chemicals used in vanadium and uranium removal do not add any substantial contaminant burden to the phosphoric acid.
There are two types of processes which can be used to extract vanadium and/or uranium values from wet-process phosphoric acid into an organic phase. In one type of process, the metal ions in a reduced state are extracted from wet-process acid into an organic phase containing esters of orthophosphoric acid as described in Hurst, et al., U.S. Pat. No. 3,835,214. In the alternative type of process the metal ions in an oxidized state are extracted into an organic phase containing a disubstituted ester of orthophosphoric acid together with a triorganophosphine oxide as described in Hurst, et al., U.S. Pat. No. 3,711,591 and Lucid, et al., U.S. Pat. No. 4,212,849. Examples of mixtures of phosphorus compounds which have been disclosed for extracting oxidized forms of the metal ions are di(2-ethylhexyl)phosphoric acid (DEPA) combined with trioctylphosphine oxide (TOPO), and dioctylphenyl phosphoric acid (DOPPA) combined with trioctylphosphine oxide. The methods of extracting oxidized froms of the metal ions from wet-process phosphoric acid into organic solutions of these mixtures of phosphorus compounds are hereinafter referred to as the DEPA-TOPO and DOPPA-TOPO processes, respectively. The present invention is an improvement in the DEPA-TOPO and DOPPA-TOPO processes for recovering vanadium from wet-process phosphoric acid.
In the DEPA-TOPO and DOPPA-TOPO processes, uranium (IV) and vanadium (IV) in the crude phosphoric acid solution are oxidized to uranium (VI) and vanadium (V) by an appropriate oxidizing agent or combination of oxidizing agents. Many oxidizing agents have been described in the art, e.g., sodium chlorate, air, pure oxygen, and hydrogen peroxide. Although sodium chlorate appears to have been extensively used, chlorides resulting from use of this oxidant are corrosive and also can carry over into the fertilizer as undesirable impurities. Air and pure oxygen have low solubility in wet-process phosphoric acid, and specialized equipment, capable of withstanding pressure, may be necessary for efficient use of these gaseous oxidants. Hydrogen peroxide is becoming increasingly desirable as an oxidant for vanadium because it is non-corrosive, contributes no impurities to the phosphate fertilizer product, and is easily mixed with the wet-process phosphoric acid solution in standard equipment.
In the art of the DEPA-TOPO and DOPPA-TOPO processes, the teaching is to first oxidize vanadium and uranium in wet-process phosphoric acid to vanadium (V) and uranium (VI). In a second, separate step, the oxidized solution is transferred to an extractor containing the organic phase. When hydrogen peroxide is used to oxidize the vanadium component of wet-process phosphoric acid, this aforesaid two-step process of oxidation and extraction leads to less than optimal recovery of vanadium and leads to inefficient use of hydrogen peroxide.
The present invention of combining the oxidation and extraction procedures into a single step will increase the recovery of vanadium, and can increase the efficiency of use of the hydrogen peroxide oxidant. Also, this combination of steps into a single step process allows for capital and operating cost savings.